Solid state diodes are used in a variety of circuit applications operating in the radio frequency range. Specifically, pin diodes and varactor diodes are used for switching purposes, as well as circuit tuning components in circuits which operate in the microwave frequency range. Varactor diodes are, for instance, commonly used to implement a voltage controlled tuning function, such as the setting of the frequency of a voltage controlled oscillator.
One of the component characteristics which limits the performance of varactor diodes and pin diodes is the series resistance which the diode offers to a radio frequency signal. In the case of a varactor diode, the Q established by a resonant circuit using the diode is highly dependent upon the series resistance of the varactor diode. In pin diode applications, the power loss incurred when switching RF signals is a significant function of the series resistance of the pin diode when biased into the conducting mode.
The series resistance of these devices in a silicided polysilicon base bipolar transistor technology is a function of the current flow from the edges of the diode active area toward the center of the diode. In the prior art approach for implementing a varactor diode or pin diode, narrow fingers of active base-collector areas, interspersed with cathode contacts on either side, are implemented in a base collector region of a silicided polysilicon base transistor. The long narrow fingers of active base collector area provide scalable area, and a relatively low series resistance since the width of the active region is kept as small as possible. However, the long, narrow diode fingers present a series resistance which is not at a minimum. It has been determined that the minimum series resistance is obtained when the perimeter to area ratio of the base collector region is minimized, and the long fingers increase the resistance associated with the diode active region. The series resistance down the length of the anode and cathode contacts also cause current crowding along the length, contributing to the resistance of the device. Accordingly, the present invention is directed to a high frequency diode which has been designed to have a minimum series resistance.